Upper Mantle Seismic Structure Beneath Southern Africa: Constraints on the Buoyancy Supporting the African Superswell |
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| Authors: | Email author" target="_blank">Martin?B?C?BrandtEmail author Stephen?P?Grand Andrew?A?Nyblade Paul?H?G?M?Dirks |
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| Institution: | (1) Seismology Unit, Council for Geoscience, Private Bag X112, Pretoria, 0001, South Africa;(2) School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa;(3) Jackson School of Geosciences, University of Texas at Austin, Austin, TX, USA;(4) Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania, USA;(5) School of Earth and Environmental Sciences, James Cook University, Townsville, Australia |
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| Abstract: | We present new one-dimensional SH-wave velocity models of the upper mantle beneath the Kalahari craton in southern Africa
obtained from waveform inversion of regional seismograms from an Mw = 5.9 earthquake located near Lake Tanganyika recorded
on broadband seismic stations deployed during the 1997–1999 Southern African Seismic Experiment. The velocity in the lithosphere
beneath the Kalahari craton is similar to that of other shields, and there is little evidence for a significant low velocity
zone beneath the lithosphere. The lower part of the lithosphere, from 110 to 220 km depth, is slightly slower than beneath
other shields, possibly due to higher temperatures or a decrease in Mg number (Mg#). If the slower velocities are caused by
a thermal anomaly, then slightly less than half of the unusually high elevation of the Kalahari craton can be explained by
shallow buoyancy from a hot lithosphere. However, a decrease in the Mg# of the lower lithosphere would increase the density
and counteract the buoyancy effect of the higher temperatures. We obtain a thickness of 250 ± 30 km for the mantle transition
zone, which is similar to the global average, but the velocity gradient between the 410 and 660 km discontinuities is less
steep than in global models, such as PREM and IASP91. We also obtain velocity jumps of between 0.16 ± 0.1 and 0.21 ± 0.1 km/s
across the 410 km discontinuity. Our results suggest that there may be a thermal or chemical anomaly in the mantle transition
zone, or alternatively that the shear wave velocity structure of the transition zone in global reference models needs to be
refined. Overall, our seismic models provide little support for an upper mantle source of buoyancy for the unusually high
elevation of the Kalahari craton, and hence the southern African portion of the African Superswell. |
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